In recent years, a demand for achieving high strength of steel sheets used for automobiles or construction machines and various parts and structures of other construction·civil engineering structures, and so on has been increasing. Against such a background a high-strength steel sheet having a maximum tensile strength of 900 MPa or more has been used mainly for reinforcing materials of bumpers, impact beams, and so on of automobiles.Further, the steel sheets used for them are normally required to have excellent corrosion resistance because they are often used outside.
As such steel sheets to be used in a field necessary for corrosion resistance, a hot-dip galvanized steel sheet obtained by performing hot-dip galvanizing on the surface of a base steel sheet has been widely used. Further, recently, there has also been widely used an alloyed hot-dip galvanized steel sheet obtained by performing, after the hot-dip galvanizing, an alloying treatment in which a plating layer is heated to a temperature equal to or higher than the melting point of Zn to diffuse Fe into the plating layer from the inside of the base steel sheet, to thereby turn the plating layer into a layer mainly composed of a Zn—Fe alloy.
By the way, when a high-strength steel sheet is applied to an automobile or the like, it is necessary to solve a problem of occurrence of delayed fracture.
The delayed fracture is a phenomenon that when working or assembling a member, cracking or a fracture does not occur, but while the member is in use under a situation where high stress acts, a fracture such as cracking occurs suddenly in an embrittling manner with hardly causing plastic deformation in external appearance. The delayed fracture has been known to be closely related to hydrogen to enter a steel sheet from the outside environment of the steel sheet. That is, the delayed fracture has been generally thought to be an embrittlement phenomenon ascribable to hydrogen to enter from the outside environment to be diffused in steel.
As a factor greatly affecting the delayed fracture, steel sheet strength has been known. This is because as the steel sheet is higher in strength, it has a higher possibility to be used in an environment where high stress acts. That is, when a low-strength material is used for a member on which high stress acts, the material is immediately plastically deformed to be fractured, so that the delayed fracture does not occur normally. On the other hand, plastic deformation and fracture do not easily occur in a high-strength material, so that a high-strength material is often used in an environment where high stress acts. Further, in a steel product to be used after being subjected to forming work such as an automobile part, residual stress occurs by the work. This residual stress increases as steel sheet strength becomes higher. Therefore, in addition to the stress by external loading, large residual stress is added to the steel sheet, and thus the delayed fracture becomes likely to occur. As a result, as the material is higher in strength, there is increased concern about occurrence of the delayed fracture.
On the other hand, a thin steel sheet, for example, a thin steel sheet having a sheet thickness of about 3.0 mm or less has been known to have anisotropy in delayed fracture resistance. That is, there is sometimes caused a difference in the delayed fracture resistance depending on a working direction (generally, a rolling direction in final cold rolling, or a rolling width direction perpendicular to it) in a manufacturing process of the steel sheet. This tendency becomes significant in a thin sheet in particular. Thus, when a high-strength thin steel sheet is used for a member on which high stress acts, taking measures for securing safety has been performed. That is, measures such that a design is made so as not to cause delayed fracture also in the direction in which the delayed fracture resistance is the poorest or the direction in which the steel sheet is applied to a member is considered so that working in the direction in which the delayed fracture resistance is poor may become slight have been taken. However, such measures cause a problem that significant restriction is placed when using the steel sheet.
Thus, as a property of the thin steel sheet itself, the development of a thin steel sheet in which not only is delayed fracture resistance improved simply, but also anisotropy of the delayed fracture resistance is reduced is strongly desired,
By the way, regarding conventional techniques related to the anisotropy of a thin steel sheet, the following techniques exist. First, as a means of reducing anisotropy of ductility to improve properties of a steel sheet, a technique illustrated in Patent Literature 1 exists. Further, as a means of reducing anisotropics of bendability and toughness to improve properties of a steel sheet, a technique illustrated in Patent Literature 2 exists. However, in both Patent Literatures 1 and 2, the delayed fracture resistance is not described, and the means for eliminating anisotropy of delayed fracture resistance is also not disclosed.
Further, in Patent Literature 3, there has been described a steel sheet having excellent delayed fracture resistance and having small anisotropics of tensile strength and ductility. However, the anisotropy of delayed fracture resistance is not described, and the means for reducing anisotropy of delayed fracture resistance is also not disclosed.
Further, as a method of improving delayed fracture resistance of a steel sheet, in Patent Literature 4 and Patent Literature 5, there has been described a steel sheet in which the main phase of the steel sheet is turned into hard, structures such as bainite, bainitic ferrite, martensite, and tempered martensite to thereby improve delayed fracture resistance. Further, in Patent Literature 6, there has been described a steel sheet in which the main phase of the steel sheet is turned into tempered marten site and then in the tempered martensite, fine carbide is dispersed to thereby improve delayed fracture resistance.
However, in all the steel sheets by these techniques of Patent Literatures 4 to 6, the structure that is hard and poor in ductility is set as the main phase, so that the ductility is poor also in the entire steel sheet, resulting in that it is unsuitable for use in which a steel sheet is subjected to heavy forming work to be used.
In Patent literature 7, there has been described that in a surface layer within 10 μm from the surface of a steel sheet, oxides are dispersed and the oxides trap hydrogen to thereby improve delayed fracture resistance of the steel sheet. Further, in Patent literature 8, there has been described a steel sheet on which the main phase of the steel sheet is turned into ferrite, marten site being a hard structure is dispersed in the steel sheet, and by fine precipitates such as Ti, Nb, and V, a block size of the martensite is made fine to thereby improve delayed fracture resistance. Further, in Patent Literature 9, there has been described a steel sheet in which in addition to making the above-described block size fine, a decarburized layer having a thickness of 0.5 μm or more is formed in a surface layer of the steel sheet to thereby improve delayed fracture resistance.
In Patent Literatures 7 to 9, there has been described that the delayed fracture resistance is improved in addition to strength and ductility, but no attention has been completely paid to the anisotropy of delayed fracture resistance.